Centre for Eye Health, NSW
The past two decades have witnessed an unprecedented explosion of innovation in eye imaging. These technologies have transformed the humble, routine eye examination into a plethora of tests integrating new technologies, such as optical coherence tomography (OCT), scanning laser ophthalmoscopy and fundus autofluorescence.
These techniques complement traditional methods of detecting structural signs of early disease and consequently may provide earlier and more accurate disease detection; still, the full utility of these tools may be under-appreciated.
Fundus autofluorescence (FAF) is an advanced imaging modality that provides insight into the health of the retinal pigment epithelium (RPE). The FAF signal derives predominantly from lipofuscin granules in the RPE, which accumulate as by-products of incomplete degradation of photoreceptor outer segments.1,2 Consequently, abnormal patterns can be interpreted as signs of current or future oxidative stress.
Excessive accumulation of lipofuscin is also recognised as a common pathogenetic pathway in various retinal diseases, including age-related macular degeneration (AMD), which precedes photoreceptor degeneration.
Alternative locations for FAF in the outer retina,3,4 photoreceptor outer segment5 and subretinal space4 and sources, such as macrophages in the new vessel complex,5 choroidal components3 and the sclera, have also been suggested. Consequently, FAF imaging provides complementary and at times, unique information compared to other modalities such as colour fundus photography).3,6-8
As in the case that follows, FAF may often reveal signs of marked retinal dysfunction in areas that appear normal on funduscopy.
A 72-year-old Caucasian female was referred to Centre for Eye Health for a macular assessment based on symptoms of an intermittent ‘dark blob’ in her central vision, most noticeable at night. She had been diagnosed with dry AMD about one year previously and had a medical history of chronic obstructive pulmonary disorder, hypertension, sleep apnoea and was a former smoker. She took the medications Cartia (aspirin) and Betaloc (metoprolol) regularly. Her mother had glaucoma.
Aided visual acuities were 6/9.5+1 OD and 6/15+2 OS and pinhole acuities were 6/9.5 OD and 6/9.5-2 OS. Contrast sensitivities were within normal limits in each eye at 1.60 log units OD and 1.64 log units OS using the MARS test (normal range 1.52-1.76 log units for patients over the age of 60 years). Amsler grid testing revealed the appearance of ‘kinked’ and mildly jagged lines inferior to fixation and in the upper left corner OD; a small area of displaced lines temporal to fixation were also perceived OS. Anterior eye examination was unremarkable.
Funduscopy, retinal photography and OCT revealed small to large, calcified (crystalline and glistening) macular drusen OU. Geographic atrophy and pigment clumping were also present in the parafoveal region OU. FAF imaging using the Spectralis HRA2 revealed distinct multilobular, extrafoveal areas of hypo-autofluorescence representing geographic atrophy OU, more numerous OD than OS, and a surrounding pattern of ‘diffuse trickling’ abnormal autofluorescence (Figures 1 and 2, top row).
Figure-1. Imaging results from the patient’s right eye, obtained at baseline (top) and 12 months later (bottom), showing progression in the areas of geographic atrophy including confluence of adjacent areas (arrowheads) and enlargement of existing areas (arrows). The changes were best visualised using FAF. The baseline FAF image also shows a ‘diffuse trickling’ or seeping pattern (asterisk) surrounding the areas of geographic atrophy, which has been associated with an eight-fold higher rate of enlargement than other presentations. CFP: colour fundus photography; FAF: fundus autofluorescence; OCT: optical coherence tomography
Figure-2. Imaging results from the patient’s left eye. The black arrowheads show development of a new hypo-autofluorescent area of geographic atrophy, which was confirmed using OCT (white arrows). The follow-up OCT line scan shows the area of geographic atrophy as focal drop-out of the ellipsoid zone and external limiting membrane, and thinning of the outer nuclear layer and RPE. There is increased signal penetration posteriorly and associated collapse of the overlying retinal layers (white arrowhead). As with the right eye, the baseline FAF image also shows a ‘diffuse trickling’ pattern (asterisk) surrounding the areas of geographic atrophy. CFP: colour fundus photography; FAF: fundus autofluorescence; OCT: optical coherence tomography
The patient was diagnosed with bilateral advanced (atrophic) AMD. A review consultation with subsequent imaging was recommended in six months. Other pertinent management considerations included interim daily self-monitoring with an Amsler grid, and the role of AREDS 2 supplements and a healthy diet in reducing the risk of progression.
Follow-up testing one year later revealed no significant change in visual acuities (6/9.5+1 OD and 6/12 OS that improved to 6/9.5+1 OD and 6/12+2 OS with pinhole). Contrast sensitivity was apparently reduced in each eye compared to baseline at 1.44 units OD and 1.40 units OS with the MARS test.
The patient denied noticing any changes on the Amsler grid through self-monitoring. Imaging showed progression of the areas of geographic atrophy including enlargement of existing areas OU, confluence of adjacent areas OD and the development of new areas OU. The progression was best visualised using FAF (Figures 1 and 2, bottom row).
Our case study highlights several applications of FAF imaging. FAF images can be obtained clinically using a scanning laser ophthalmoscope1 such as the Spectralis Heidelberg Retina Angiograph HRA classic or HRA2 (Heidelberg Engineering, Heidelberg, Germany) or Optomap ultra-widefield imaging (Optos, Dunfermline, Scotland, UK).
Alternatively, clinical FAF images may be obtained more inexpensively using a commercially-available fundus camera and standardised filters.9 However, FAF images obtained using a fundus camera may be inferior, of lower contrast, with higher background noise and more susceptible to degradation by media opacities, compared to a scanning laser ophthalmoscope10,11 owing to indirect light captured from structures outside of the plane of focus, which is unavoidable in a flash photography system.
Wavelength variations between the systems are also likely to cause subtle differences in the final FAF image and associated appearance of lesions; however, the clinical findings overall are still likely to be comparable11,12 and images from different systems can be interpreted using the same framework of evidence.
The final output of FAF imaging appears as an en face, ‘front on’, image of naturally or pathologically occurring fluorophores, which allows the clinician to visualise the spatial distribution and intensity of autofluorescence across the fundus. The field of view may vary between 30 degrees and 200 degrees, depending on the instrument employed. At present, clinical evaluation of case images is primarily performed qualitatively and may require post-acquisition adjustments in brightness and contrast.13
The imaging results of this case correlated consistently with other functional and structural findings,14 such as the patient’s chief complaint and OCT. In particular, FAF allowed for a refined diagnosis and provided a better indication of the extent of retinal dysfunction than OCT or other modalities.
In AMD, areas of geographic atrophy may be difficult to identify using funduscopy or retinal photography alone due to variations in fundus pigmentation and the presence of small, satellite lesions. Had the extent of these areas been underestimated, the true diagnosis of advanced AMD may have been misclassified as intermediate.
In this case, FAF also enabled the identification of a high-risk phenotype, characterised by the ‘diffuse trickling’8 signature in the junctional zone surrounding the atrophic area. The clinical implication of this finding is that the patient needs to be more closely monitored for the potential development of choroidal neovascularisation.
Finally, follow-up FAF imaging allowed for disease progression to be more easily recognised and documented. This information is directly relevant clinically and may be applied to better educate the patient about their increased risk of progression to the exudative form of advanced AMD and to encourage compliance with clinical recommendations, for example, to reinforce the importance of daily self-monitoring with an Amsler grid.
Accordingly, the reasons for using advanced ocular imaging (including FAF) can be summarised as follows:
- to detect ocular disease at an earlier stage, particularly when the results correlate with other structural or functional tests
- to aid in the differential diagnosis of ocular diseases and to specifically stratify phenotypes or stages of disease in order to identify patients at risk of visual impairment
- for closer follow-up of cases or to measure the response to treatment
- for better photo-documentation, patient education and management.
The principles of FAF imaging described in this article using a case of atrophic AMD can also be extrapolated to eye diseases other than AMD.15
There has been a clinical paradigm shift toward the inclusion of advanced imaging into routine primary eye care. In the future, we can expect to see a refined understanding and a growth in the collective wisdom regarding the utility of instruments such as FAF in disease detection, stratification and differential diagnosis.
As eye-care professionals, we face the challenge of keeping pace with the evidence base surrounding these new technologies in order to optimally manage our patients. FAF imaging represents a rapid, effective, non-invasive imaging method with significant and often under-estimated applicability.
The author thanks Paula Katalinic, Professor Michael Kalloniatis and Michael Yapp for reviewing the article.
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